Physiology

Cholesterol is essential for all animal life, with each cell capable of synthesizing it by way of a complex 37-step process. This begins with the mevalonate or HMG-CoA reductase pathway, the target of statin drugs, which encompasses the first 18 steps. This is followed by 19 additional steps to convert the resulting lanosterol into cholesterol.

A human male weighing 68 kg (150 lb) normally synthesizes about 1 gram (1,000 mg) of cholesterol per day, and his body contains about 35 g, mostly contained within the cell membranes. Typical daily cholesterol dietary intake for a man in the United States is 307 mg.[9]

Most ingested cholesterol is esterified, which causes it to be poorly absorbed by the gut. The body also compensates for absorption of ingested cholesterol by reducing its own cholesterol synthesis.[10] For these reasons, cholesterol in food, seven to ten hours after ingestion, has little, if any effect on concentrations of cholesterol in the blood. However, during the first seven hours after ingestion of cholesterol, as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins (which transport all fats in the water outside cells), the concentrations increase.[11]

Cholesterol is recycled in the body. The liver excretes it in a non-esterified form (via bile) into the digestive tract. Typically, about 50% of the excreted cholesterol is reabsorbed by the small intestine back into the bloodstream.[12]

Plants make cholesterol in very small amounts.[13] Plants manufacture phytosterols (substances chemically similar to cholesterol), which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption.[14] When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism. The intake of naturally occurring phytosterols, which encompass plant sterols and stanols, ranges between ≈200–300 mg/day depending on eating habits.[15] Specially designed vegetarian experimental diets have been produced yielding upwards of 700 mg/day.[16]

Function

Cholesterol, given that it composes about 30% of all animal cell membranes, is required to build and maintain membranes and modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group of each cholesterol molecule interacts with the water molecules surrounding the membrane as do the polar heads of the membranephospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolarfatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains, cholesterol increases membrane packing, which both alters membrane fluidity[17] and maintains membrane integrity so that animal cells do not need to build cell walls (like plants and most bacteria). The membrane remains stable and durable without being rigid, allowing animal cells to change shape and animals to move.

The structure of the tetracyclic ring of cholesterol contributes to the fluidity of the cell membrane, as the molecule is in a trans conformation making all but the side chain of cholesterol rigid and planar.[18] In this structural role, cholesterol also reduces the permeability of the plasma membrane to neutral solutes,[19]hydrogen ions, and sodium ions.[20]

Within the cell membrane, cholesterol also functions in intracellular transport, cell signaling and nerve conduction. Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including caveola-dependent and clathrin-dependent endocytosis. The role of cholesterol in endocytosis of these types can be investigated by using methyl beta cyclodextrin (MβCD) to remove cholesterol from the plasma membrane. Recent studies show that cholesterol is also implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane, which brings receptor proteins in close proximity with high concentrations of second messenger molecules.[21] In multiple layers, cholesterol and phospholipids, both electrical insulators, can facilitate speed of transmission of electrical impulses along nerve tissue. For many neuron fibers, a myelin sheath, rich in cholesterol since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses.[22]Demyelination (loss of some of these Schwann cells) is believed to be part of the basis for multiple sclerosis.

The liver excretes cholesterol into biliary fluids, which is then stored in the gallbladder. Bile contains bile salts, which solubilize fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat-soluble vitamins, A, D, E, and K.[24]

Biosynthesis and regulation

Biosynthesis

All animal cells manufacture cholesterol, for both membrane structure and other uses, with relative production rates varying by cell type and organ function. About 80% of total daily cholesterol production occurs in the liver and the intestines;[25] other sites of higher synthesis rates include adrenal glands, and reproductive organs.

This molecule is then reduced to mevalonate by the enzyme HMG-CoA reductase. Production of mevalonate is the rate-limiting and irreversible step in cholesterol synthesis and is the site of action for statins (a class of cholesterol lowering drugs).

Mevalonate is finally converted to isopentenyl pyrophosphate (IPP) through two phosphorylation steps and one decarboxylation step that requires ATP.

Three molecules of isopentenyl pyrophosphate condense to form farnesyl pyrophosphate through the action of geranyl transferase.

Regulation of cholesterol synthesis

Biosynthesis of cholesterol is directly regulated by the cholesterol levels present, though the homeostatic mechanisms involved are only partly understood. A higher intake from food leads to a net decrease in endogenous production, whereas lower intake from food has the opposite effect. The main regulatory mechanism is the sensing of intracellular cholesterol in the endoplasmic reticulum by the proteinSREBP (sterol regulatory element-binding protein 1 and 2).[30] In the presence of cholesterol, SREBP is bound to two other proteins: SCAP (SREBP cleavage-activating protein) and INSIG-1. When cholesterol levels fall, INSIG-1 dissociates from the SREBP-SCAP complex, which allows the complex to migrate to the Golgi apparatus. Here SREBP is cleaved by S1P and S2P (site-1 protease and site-2 protease), two enzymes that are activated by SCAP when cholesterol levels are low.

The cleaved SREBP then migrates to the nucleus, and acts as a transcription factor to bind to the sterol regulatory element (SRE), which stimulates the transcription of many genes. Among these are the low-density lipoprotein (LDL) receptor and HMG-CoA reductase. The LDL receptor scavenges circulating LDL from the bloodstream, whereas HMG-CoA reductase leads to an increase of endogenous production of cholesterol.[31] A large part of this signaling pathway was clarified by Dr. Michael S. Brown and Dr. Joseph L. Goldstein in the 1970s. In 1985, they received the Nobel Prize in Physiology or Medicine for their work. Their subsequent work shows how the SREBP pathway regulates expression of many genes that control lipid formation and metabolism and body fuel allocation.

Cholesterol synthesis can also be turned off when cholesterol levels are high. HMG-CoA reductase contains both a cytosolic domain (responsible for its catalytic function) and a membrane domain. The membrane domain senses signals for its degradation. Increasing concentrations of cholesterol (and other sterols) cause a change in this domain’s oligomerization state, which makes it more susceptible to destruction by the proteosome. This enzyme’s activity can also be reduced by phosphorylation by an AMP-activated protein kinase. Because this kinase is activated by AMP, which is produced when ATP is hydrolyzed, it follows that cholesterol synthesis is halted when ATP levels are low.[32]

Plasma transport and regulation of absorption

Lipid logistics: transport of triglycerides and cholesterol in organisms in form of lipoproteins as chylomicrons, VLDL, LDL, IDL, HDL.

As an isolated molecule, cholesterol is only minimally soluble in water, or hydrophilic. Because of it this, it dissolves in blood at exceedingly small concentrations. To be transported effectively, cholesterol is instead packaged within lipoproteins, complex discoidal particles with exterior amphiphilic proteins and lipids, whose outward-facing surfaces are water-soluble and inward-facing surfaces are lipid-soluble. This allows it to travel through the blood via emulsification. Unbound cholesterol, being amphipathic, is transported in the monolayer surface of the lipoprotein particle along with phospholipids and proteins. Cholesterol esters bound to fatty acid, on the other hand, are transported within the fatty hydrophilic core of the lipoprotein, along with triglyceride.[33]

Lipoprotein particles are organized by complex apolipoproteins, typically 80–100 different proteins per particle, which can be recognized and bound by specific receptors on cell membranes, directing their lipid payload into specific cells and tissues currently ingesting these fat transport particles. These surface receptors serve as unique molecular signatures, which then help determine fat distribution delivery throughout the body.[33]

Chylomicrons, the least dense cholesterol transport molecules, contain apolipoprotein B-48, apolipoprotein C, and apolipoprotein E (the principal cholesterol carrier in the brain[34]) in their shells. Chylomicrons carry fats from the intestine to muscle and other tissues in need of fatty acids for energy or fat production. Unused cholesterol remains in more cholesterol-rich chylomicron remnants, and taken up from here to the bloodstream by the liver.[33]

VLDL molecules are produced by the liver from triacylglycerol and cholesterol which was not used in the synthesis of bile acids. These molecules contain apolipoprotein B100 and apolipoprotein E in their shells, and can be degraded by lipoprotein lipase on the artery wall to IDL. This arterial wall cleavage allows absorption of triacylglycerol and increases concentration of circulating cholesterol. IDL molecules are then consumed in two processes: half is metabolized by HTGL and taken up by the LDL receptor on the liver cell surfaces, while the other half continues to lose triacylglycerols in the bloodstream until they become cholesterol laden LDL particles.[33]

LDL particles are the major blood cholesterol carriers. Each one contains approximately 1,500 molecules of cholesterol ester. LDL molecule shells contain just one molecule of apolipoprotein B100, recognized by LDL receptors in peripheral tissues. Upon binding of apolipoprotein B100, many LDL receptors concentrate in clathrin-coated pits. Both LDL and its receptor form vesicles within a cell via endocytosis. These vesicles then fuse with a lysosome, where the lysosomal acid lipase enzyme hydrolyzes the cholesterol esters. The cholesterol can then be used for membrane biosynthesis or esterified and stored within the cell, so as to not interfere with the cell membranes.[33]

LDL receptors are used up during cholesterol absorption, and its synthesis is regulated by SREBP, the same protein that controls the synthesis of cholesterol de novo, according to its presence inside the cell. A cell with abundant cholesterol will have its LDL receptor synthesis blocked, to prevent new cholesterol in LDL molecules from being taken up. Conversely, LDL receptor synthesis proceeds when a cell is deficient in cholesterol.[33]

When this process becomes unregulated, LDL molecules without receptors begin to appear in the blood. These LDL molecules are oxidized and taken up by macrophages, which become engorged and form foam cells. These foam cells often become trapped in the walls of blood vessels and contribute to atherosclerotic plaque formation. Differences in cholesterol homeostasis affect the development of early atherosclerosis (carotid intima-media thickness).[35] These plaques are the main causes of heart attacks, strokes, and other serious medical problems, leading to the association of so-called LDL cholesterol (actually a lipoprotein) with “bad” cholesterol.[32]

HDL particles are thought to transport cholesterol back to the liver, either for excretion or for other tissues that synthesize hormones, in a process known as reverse cholesterol transport (RCT).[36] Large numbers of HDL particles correlates with better health outcomes,[37] whereas low numbers of HDL particles is associated with atheromatous disease progression in the arteries.[citation needed]

Metabolism, recycling and excretion

Cholesterol is susceptible to oxidation and easily forms oxygenated derivatives called oxysterols. Three different mechanisms can form these: autoxidation, secondary oxidation to lipid peroxidation, and cholesterol-metabolizing enzyme oxidation. A great interest in oxysterols arose when they were shown to exert inhibitory actions on cholesterol biosynthesis.[38] This finding became known as the “oxysterol hypothesis”. Additional roles for oxysterols in human physiology include their participation in bile acid biosynthesis, function as transport forms of cholesterol, and regulation of gene transcription.[39]

In biochemical experiments radiolabelled forms of cholesterol, such as tritiated-cholesterol are used. These derivatives undergo degradation upon storage and it is essential to purify cholesterol prior to use. Cholesterol can be purified using small Sephadex LH-20 columns.[40]

Cholesterol is oxidized by the liver into a variety of bile acids.[41] These, in turn, are conjugated with glycine, taurine, glucuronic acid, or sulfate. A mixture of conjugated and nonconjugated bile acids, along with cholesterol itself, is excreted from the liver into the bile. Approximately 95% of the bile acids are reabsorbed from the intestines, and the remainder are lost in the feces.[42] The excretion and reabsorption of bile acids forms the basis of the enterohepatic circulation, which is essential for the digestion and absorption of dietary fats. Under certain circumstances, when more concentrated, as in the gallbladder, cholesterol crystallises and is the major constituent of most gallstones (lecithin and bilirubin gallstones also occur, but less frequently).[43] Every day, up to 1 g of cholesterol enters the colon. This cholesterol originates from the diet, bile, and desquamated intestinal cells, and can be metabolized by the colonic bacteria. Cholesterol is converted mainly into coprostanol, a nonabsorbable sterol that is excreted in the feces.[citation needed]

Although cholesterol is a steroid generally associated with mammals, the human pathogen Mycobacterium tuberculosis is able to completely degrade this molecule and contains a large number of genes that are regulated by its presence.[44] Many of these cholesterol-regulated genes are homologues of fatty acidβ-oxidation genes, but have evolved in such a way as to bind large steroid substrates like cholesterol.[45][46]

Plant cells synthesize cholesterol as a precursor for other compounds, such as phytosterols and steroidalglycoalkaloids, with cholesterol remaining in plant foods only in minor amounts or absent.[48][50] Some plant foods, such as avocado, flax seeds and peanuts, contain phytosterols, which compete with cholesterol for absorption in the intestines, reduce the absorption of both dietary and bile cholesterol.[51] A typical diet contributes on the order of 0.2 gram of phytosterols, which is not enough to have a significant impact on blocking cholesterol absorption. Phytosterols intake can be supplemented through the use of phytosterol-containing functional foods or dietary supplements that are recognized as having potential to reduce levels of LDL-cholesterol.[52] Some supplemental guidelines have recommended doses of phytosterols in the 1.6–3.0 grams per day range (Health Canada, EFSA, ATP III, FDA). A recent meta-analysis demonstrating a 12% reduction in LDL-cholesterol at a mean dose of 2.1 grams per day.[53] However, the benefits of a diet supplemented with phytosterols have been questioned.[54]

In 2016, the United States Department of Agriculture Dietary Guidelines Advisory Committee recommended that Americans eat as little dietary cholesterol as possible.[55] Increased dietary intake of industrial trans fats is associated with an increased risk in all-cause mortality and cardiovascular diseases.[56]Trans fats have been shown to correlate with reduced levels of HDL and increased levels of LDL.[57] Based on this evidence, along with other claims implicating low HDL and high LDL levels in cardiovascular disease, many health authorities advocate reducing LDL-cholesterol through changes in diet in addition to other lifestyle modifications.[58] The related studies which correlate trans fats, as well as saturated fats, with unhealthy serum cholesterol levels, have since been contested on numerous points. The most notable and egregious challenge to these standards comes from a NCBI published meta analysis of the data used in the development of these guidelines, in which the correlation between serum cholesterol and saturated fat intake, was similarly or less significant than the correlation to visceral fat.[59] As well as others, one of which concluded that current evidence “does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats.”[60] Other evidences such as metabolic ward and lab studies, including a study where rats subjected to high-fat or fructose diets became dyslipidemic[61] are similarly questionable, given indications of an increase of produced visceral fat, which occurs as a result of metabolic differences in the processing of fructose.[62] A general inconsistency of conclusions regarding the impact of simple carbohydrates on visceral fat, and a lack of data regarding the causal relationship between serum cholesterol and either saturated fat and visceral fat, makes drawing a definitive conclusion unreasonable, especially given the presence of numerous correlations. As such, given that well designed, adequately powered randomized controlled trials investigating patient-relevant outcomes of low-fat diets for otherwise healthy people with hypercholesterolaemia are lacking; large, parallel, randomized controlled trials are still needed to investigate the effectiveness of a cholesterol-lowering diet and the addition of omega-3 fatty acids, soya protein, plant sterols or stanols, especially in the case of familial hypercholesterolemia.[63][64]

Clinical significance

Hypercholesterolemia

Cholesterolemia and mortality for men and women <50 years and >60 years

According to the lipid hypothesis, elevated levels of cholesterol in the blood lead to atherosclerosis which may increase the risk of heart attack, stroke, and peripheral vascular disease. Since higher blood LDL – especially higher LDL concentrations and smaller LDL particle size – contributes to this process more than the cholesterol content of the HDL particles,[68] LDL particles are often termed “bad cholesterol”. High concentrations of functional HDL, which can remove cholesterol from cells and atheromas, offer protection and are commonly referred to as “good cholesterol”. These balances are mostly genetically determined, but can be changed by body composition, medications, diet,[69] and other factors.[70] A 2007 study demonstrated that blood total cholesterol levels have an exponential effect on cardiovascular and total mortality, with the association more pronounced in younger subjects. Because cardiovascular disease is relatively rare in the younger population, the impact of high cholesterol on health is larger in older people.[71]

Elevated levels of the lipoprotein fractions, LDL, IDL and VLDL, rather than the total cholesterol level, correlate with the extent and progress of atherosclerosis.[72] Conversely, the total cholesterol can be within normal limits, yet be made up primarily of small LDL and small HDL particles, under which conditions atheroma growth rates are high. A post hoc analysis of the IDEAL and the EPIC prospective studies found an association between high levels of HDL cholesterol (adjusted for apolipoprotein A-I and apolipoprotein B) and increased risk of cardiovascular disease, casting doubt on the cardioprotective role of “good cholesterol”.[73][74]

Elevated cholesterol levels are treated with a strict diet consisting of low saturated fat, trans fat-free, low cholesterol foods,[75][76] often followed by one of various hypolipidemic agents, such as statins, fibrates, cholesterol absorption inhibitors, nicotinic acid derivatives or bile acid sequestrants.[77] There are several international guidelines on the treatment of hypercholesterolaemia.[78]

Human trials using HMG-CoA reductase inhibitors, known as statins, have repeatedly confirmed that changing lipoprotein transport patterns from unhealthy to healthier patterns significantly lowers cardiovascular disease event rates, even for people with cholesterol values currently considered low for adults.[79] Studies have also found that statins reduce atheroma progression.[80] As a result, people with a history of cardiovascular disease may derive benefit from statins irrespective of their cholesterol levels (total cholesterol below 5.0 mmol/L [193 mg/dL]),[81] and in men without cardiovascular disease, there is benefit from lowering abnormally high cholesterol levels (“primary prevention”).[82] Primary prevention in women was originally practiced only by extension of the findings in studies on men,[83] since, in women, none of the large statin trials conducted prior to 2007 demonstrated a significant reduction in overall mortality or in cardiovascular endpoints.[84] Meta-analyses have demonstrated significant reductions in all-cause and cardiovascular mortality, without significant heterogeneity by sex.[85]

The 1987 report of National Cholesterol Education Program, Adult Treatment Panels suggests the total blood cholesterol level should be: < 200 mg/dL normal blood cholesterol, 200–239 mg/dL borderline-high, > 240 mg/dL high cholesterol.[86] The American Heart Association provides a similar set of guidelines for total (fasting) blood cholesterol levels and risk for heart disease:[87] Statins are effective in lowering LDL cholesterol and widely used for primary prevention in people at high risk of cardiovascular disease, as well as in secondary prevention for those who have developed cardiovascular disease.[88]

More current testing methods determine LDL (“bad”) and HDL (“good”) cholesterol separately, allowing cholesterol analysis to be more nuanced. The desirable LDL level is considered to be less than 130 mg/dL (2.6 mmol/L),[89] although a newer upper limit of 70 mg/dL (1.8 mmol/L) can be considered in higher-risk individuals based on some of the above-mentioned trials. A ratio of total cholesterol to HDL—another useful measure—of far less than 5:1 is thought to be healthier.

Reference ranges for blood tests, showing usual, as well as optimal, levels of HDL, LDL and total cholesterol in mass and molar concentrations, is found in orange color at right, that is, among the blood constituents with the highest concentration.

Total cholesterol is defined as the sum of HDL, LDL, and VLDL. Usually, only the total, HDL, and triglycerides are measured. For cost reasons, the VLDL is usually estimated as one-fifth of the triglycerides and the LDL is estimated using the Friedewald formula (or a variant): estimated LDL = [total cholesterol] − [total HDL] − [estimated VLDL]. VLDL can be calculated by dividing total triglycerides by five. Direct LDL measures are used when triglycerides exceed 400 mg/dL. The estimated VLDL and LDL have more error when triglycerides are above 400 mg/dL.[90]

In the Framingham Heart Study, in subjects over 50 years of age, they found an 11% increase overall and 14% increase in cardiovascular disease mortality per 1 mg/dL per year drop in total cholesterol levels. The researchers attributed this phenomenon to the fact that people with severe chronic diseases or cancer tend to have below-normal cholesterol levels.[91] This explanation is not supported by the Vorarlberg Health Monitoring and Promotion Programme, in which men of all ages and women over 50 with very low cholesterol were likely to die of cancer, liver diseases, and mental diseases. This result indicates the low-cholesterol effect occurs even among younger respondents, contradicting the previous assessment among cohorts of older people that this is a proxy or marker for frailty occurring with age.[92]

Although there is a link between cholesterol and atherosclerosis as discussed above,[93] a 2014 meta-analysis concluded there is insufficient evidence to support the recommendation of high consumption of polyunsaturated fatty acids and low consumption of total saturated fats for cardiovascular health.[94] A 2016 review concluded there was either no link between LDL and mortality or that lower LDL was linked to a higher mortality risk, especially in older adults.[95]

Hypocholesterolemia

Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, but some studies suggest a link with depression, cancer, and cerebral hemorrhage. In general, the low cholesterol levels seem to be a consequence, rather than a cause, of an underlying illness.[71] A genetic defect in cholesterol synthesis causes Smith–Lemli–Opitz syndrome, which is often associated with low plasma cholesterol levels. Hyperthyroidism, or any other endocrine disturbance which causes upregulation of the LDL receptor, may result in hypocholesterolemia.[96]

Cholesterol testing

The American Heart Association recommends testing cholesterol every 4–6 years for people aged 20 years or older.[97] A separate set of American Heart Association guidelines issued in 2013 indicates that patients taking statin medications should have their cholesterol tested 4–12 weeks after their first dose and then every 3–12 months thereafter.[98]

It is recommended to test cholesterol at least every five years if a person has total cholesterol of 5.2 mmol/L or more (200+ mg/dL), or if a man over age 45 or a woman over age 50 has HDL-C values less than 1 mmol/L (40 mg/dL), or there are other drivers heart disease and stroke. Additional drivers of heart disease include diabetes mellitus, hypertension (or use of anti-hypertensive drug|anti-hypertensive medication), low HDL level, family history of coronary artery disease (CAD) and hypercholesterolemia, and cigarette smoking.[99]

Interactive pathway map

Click on genes, proteins and metabolites below to link to respective articles.[§ 1]

Stereoisomers

Cholesterol has 256 stereoisomers that arise from its 8 stereocenters, although only two of the stereoisomers are of biochemical significance (nat-cholesterol and ent-cholesterol, for natural and enantiomer, respectively),[100][101] and only one occurs naturally (nat-cholesterol).

^“Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. The Expert Panel”. Arch. Intern. Med. 148 (1): 36–69. January 1988. doi:10.1001/archinte.148.1.36. PMID3422148.